Molding apparatus are known in which a resin material supplied to pots of a mold is heated and pressurized to a molten state and then injected into molding cavities through transfer channels to enclose with the resin semiconductor chips on a lead frame set in position at the cavity portion.
Such molding apparatus heretofore proposed include those of the multiplunger type which have, for example, the following construction.
The molding apparatus comprises a stationary mold, a movable mold opposed to the stationary mold, a plurality of pots formed in one of these molds for supplying a resin material, a plurality of molding cavities defined by the opposed molds, channels for holding each pot in communication with the corresponding cavities to transfer the resin material in molten state, plungers corresponding to the pots in position and number for pressurizing the resin material, a drive assembly for advancing or retracting the plungers, etc.
The plungers are attached to a plunger holder which is movable forward and backward. However, when the plungers are fixed to the holder, the resin molding operation involves the following problem.
The plungers are moved forward or backward at the same time, so that if excessive or insufficient amounts of resin material are fed to some of the pots, it is impossible to subject the portions of resin material to a uniform pressure. Consequently the resin material is melted, transferred and injected into the cavities with application of pressure under varying conditions. For example, if insufficient amounts of resin material are fed to cavities, the condition for molding the resin within the cavities varies from cavity to cavity. Under such molding conditions, pinholes or voids will occur in the surface or interior of the molded resin bodies enclosing semiconductor chips, giving lower moisture resistance and reduced reliability to the product.
To eliminate the above drawback, a molding apparatus has been proposed wherein the plungers are movable forward and backward, each independently of another in accordance with the amount of resin material fed to the corresponding pot. In this case, even if an excess or deficiency occurs in the amounts of resin material fed to the cavities, the resin within the cavities can be molded under a uniform condition.
However, even if the improved molding apparatus is used, voids or like faults are likely to occur in molded resin bodies, presumably owing to the following cause.
When the resin material fed to a pot is melted with application of pressure by the plunger, the resin material partly ingresses into the clearance around the plunger in the pot, presenting difficulty to the sliding action of the plunger. Since each plunger is made movable forward or backward axially thereof independently of another in accordance with the amount of supply of the resin material, the resin material present in the clearance is likely to impede the sliding action of the plunger irrespective of the amount of supply of the resin material. Thus, even if the resin material is fed to the pot in a specified or smaller amount, the plunger, when prevented from sliding smoothly, fails to apply the desired pressure to the resin material in the pot. The pressure applied to the resin material by the plungers is not uniform, with the result that the resin material in the cavities is molded under varying conditions.